1. Field of the Invention
This invention relates to signal demodulation during communications between a base station and a mobile module in a 3rd Generation Partnership Project wireless communications network. More specifically, architecture to properly integrate demodulation modules in a 3GPP receiver is disclosed.
2. Description of the Prior Art
A mobile unit in a wireless communications network functions in a difficult environment. Structures and terrain scatter reflect a signal transmitted from a base station to the mobile unit. As a result, the signal picked up by a receiving antenna is a sum of all the scattered and reflected, or multipath, signals. In general, the quality of this received multipath signal is affected by two major factors.
The first factor is called slow fading or lognormal fading. Slow fading results from absorption of the signal by terrain between the base station and the mobile unit. A good example of slow fading is a mobile unit moving through a tunnel, possibly resulting in loss of signal strength.
The second factor is called fast fading, multipath fading, or Rayleigh fading. Rayleigh fading results when the multipath signals arrive at the mobile unit and combine destructively, possibly causing a loss of the entire bandwidth. Another form of Rayleigh fading is a Doppler shift in frequency due to motion of the mobile unit relative to the base station.
For these reasons a typical Wideband Code Division Multiple Access (WCDMA) Universal Mobile Telecommunications System Terrestrial Radio Access Network (UTRAN) receiver requires several modules to demodulate a received signal correctly. A prior art WCDMA UTRAN receiver 110 is shown in FIG. 1. The receiver 110 comprises a Delay Estimation (DE) module 112, Rake Fingers module 115, a Maximum Ratio Combing (MRC) module 118, a Channel Estimation (CE) module 120, a Velocity Estimation (VE) module 122, and an Automatic Frequency Control 125 (AFC) module.
An channel complex gain signal from a Square-Rooted-Raised-Cosine (SRRC) filter (not shown) is transmitted to the DE 112, to the Rake Fingers 115, and to the CE 120. The output of the DE 112 is fed to another input of the Rake Fingers 115. The output of the Rake Fingers 115 then is transmitted to the MRC 118. The output of the CE is transmitted to the VE 122 and to the AFC 125. The output of the AFC 125 is transmitted back to the CE 120 and to another input of the VE 122. The output of the VE 122 is also routed back to the CE 120. Another output of the CE 120 is routed (along with the output of the Rake Fingers 115) to another input of the MRC 118 to complete the generation of the demodulated signal before Demultiplexing and Dechannel Coding (DeMCC).
The CE 120 utilizes a bandwidth filter to help estimate the channel complex gains including amplitude and phases. Bandwidth filters are well known in the art to allow predefined ranges of frequencies to pass while attenuating frequencies outside of the predefined range. Obviously the predefined range is centered on the expected transmission channel. The AFC 125 compensates for the difference in frequencies between the transmitter and the receiver due to variations in local oscillators. The VE 122 measures the velocity of a mobile unit relative to the base station. The AFC 125 and the VE 122 require the estimation results of the CE 120, but the CE 120 also needs the results of the AFC 125 and the VE 122 to work effectively. These feedback loops between the CE 120, the VE 122, and the AFC 125 prevent efficient and stable operation of the receiver 110.
For example, the bandwidth filter of Channel Estimation (CE) in the receiver 110 must be designed for the Doppler spread. This is easily illustrated. FIG. 2 shows a spectrum of channel complex gain 15 of a received signal neatly centered within a relatively large allotted bandwidth filter 10 when no frequency offset exists. FIG. 3 shows a received signals spectrum of channel complex gain 25 remaining within the large allotted bandwidth filter 10 even with a frequency offset. In sharp contrast to these views are FIG. 4 and FIG. 5 showing the same spectrum of channel complex gains 15 (FIG. 4) and 25 (FIG. 5) when a much smaller narrower bandwidth filter 20 is used. FIG. 5 shows the received signals spectrum of channel complex gain 25 falling outside of the allotted bandwidth filter 20 due to a frequency offset, distorting signals.
Note that in a WCDMA system the frequency is required to be within 0.1 ppm, which is around 200 Hz and roughly corresponds to the Doppler induced frequency spread occurring in a mobile unit traveling at 100 kph. There may be an additional frequency offset resulting from variations in local oscillators. Because the VE 122 relies on the results of the CE 120, the bandwidth filter of the CE 120 must be wide enough to allow the complex gain to pass through the CE 120 without encountering the signal distortions shown in FIG. 5, regardless of the speed of the mobile unit. However, to get better performance, the bandwidth filter of the CE 120 should be tailored to fit the Doppler spread.